Modern computer and telephone system technology have made the transmission of computer originated data over conventional telephone lines a commonplace event. In such systems, a computer is connected through a suitable interface, such an a RS 232 interface, to provide serial data signals to a conventional wire line modem. With modems of this type, when signal quality changes induce errors in the modem data stream, an ARQ (Automatic Repeat Request) or packet repeat scheme is conventionally employed for controlling these errors. This requires a complete repeat of numerous bytes of data until such time as all of the bytes of data in the packet are received correctly. However, the low frequency of signal quality change induced errors in a wire line environment makes this an efficient method of controlling error.
Current wire line modem technology provides a scrambled modulated signal to the telephone line which will not be interpreted by telephone equipment as a valid switch command. To accomplish this, conventional wire line modems are provided with a scrambler circuit which assures that the modulated signal is continuously changing. This changing signal is used by the modem PLL (Phase Locked Loop) circuitry to provide synchronization, for without this scramble modulated signal, a static condition of the modem will cause the PLL to loose synchronization and the telephone equipment to interpret the static signal as a switch command.
Conventional telephone modems have operated effectively to interface computers with a telephone system for data transmission, but these modems do not operate effectively to provide data transmission over conventional cellular telephone equipment. In a cellular telephone system, data transmission must occur to and from a moving vehicle which may be passing between zones or cells in the system. For example, a city with cellular service is divided into a plurality of adjoining geographic cells, each of which has its own transmit/receive antenna controlled by a mobile switching office. For conventional cellular telephone voice communication, an automobile travelling through a city passes from cell to cell, and the signal is transferred from antenna to antenna. This transfer process interrupts communications for a brief period, normally a fraction of a second, and does not cause a problem for voice communication. However, for data communications, this "hand-off" process results in significant problem if conventional wire line modems are used.
A wire line modem for use with normal telephone equipment will disconnect upon experiencing a carrier signal loss. Thus, such a modem, when used with a cellular telephone system, will disconnect each time the vehicle in which the modem is mounted travels between cells, for some carrier loss will always be experienced when the radio signal used for cellular telephone communication is switched between the low power transmission stations of adjacent cells.
In the cellular telephone environment, numerous errors are induced into data transmission because of the problems associated with cellular telephone communication. Echo and fading problems cause multiple bit errors in the data stream, and such problems occur frequently with a moving vehicle. For example, the transmitted signal may hit a building or other obstacle and bounce erratically or fade as the vehicle is shielded from the cell antenna. This high frequency of error in the data stream transmitted by cellular transmission renders the error correction protocol present in conventional wire line modems unsuitable for cellular use. Errors occur so frequently in a cellular environment that the number of repeat requests becomes large and data transmission efficiency is reduced below an acceptable amount. In some instances, errors may occur so often that a correct packet may never be received. Thus, the error correction protocol present in conventional telephone modems is unable to cope with the problems presented in a cellular environment.
Finally, as previously indicated, the conventional wire line telephone modem incorporates scrambler circuitry to ensure that the modulated signal is continuously changing to provide synchronization for the modem PLL circuitry. However, such scramblers employ a polynomial which has the effect of increasing the number of bit errors received. If a single bit error occurs during the transmission of data, that single bit error will be presented when received, but in addition, that error will propagate through the scrambler polynomial and later cause two additional errors in the received data presented to the user. These errors, coupled with those normally inherent with a cellular telephone system, will completely overwhelm the error correction circuitry present in a conventional modem.
In the past, systems have been developed for communicating data between a plurality of geographical zones and a host computer by means of portable radios. Such systems are disclosed in U.S. Pat. Nos. 4,525,861 and 4,545,071 to Thomas A. Freeburg. Although these patented systems effectively provide data communications from a host computer throughout a geographical area divided into zones, they do not address the problems presented by hand-off or echoing and fading in a cellular telephone system.